Continuous DPM Monitoring in Underground Mine Environments: Demonstration of Potential Options in the Laboratory and Field

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Abstract

Diesel particulate matter (DPM) is the solid portion of diesel exhaust. DPM occurs primarily in the submicron range, and poses a number of respiratory and other health hazards including cardiovascular and pulmonary disease. Underground miners typically have the highest DPM exposures compared to other occupations. This is because many mines are characterized by confined work spaces and large diesel equipment fleets. Exposures can be a particularly high hazard in large opening mines where ventilation can be challenging. As such, DPM monitoring is critical to protecting miner health and informing a range of engineering decisions.
DPM is primarily composed of two components, elemental carbon (EC) and organic carbon (OC), which are often summed to report total carbon (TC). The ratio of EC to OC, and presence of a number of other minor constituents such as sorbed metals, can vary with many factors such as engine operating conditions, maintenance, fuel types and additives, and the level and type of exhaust after-treatments used. Given its complexity, DPM cannot be measured directly, and either TC or EC are generally used as a surrogate. Currently, the Mining Safety and Health Administration (MSHA) limits personal exposures of underground metal/non-metal miners to 160 µg TC/m3 on an 8-hr time weighted average basis. Compliance is demonstrated by collecting full-shift personal filter samples, which are later analyzed using the NIOSH 5040 Standard Method. For engineering purposes, area samples can also be collected and analyzed. The typical lag time between sample collection and reporting of results is on the order of weeks, and this presents a real problem for identifying and remediating conditions that led to overexposures or high DPM in area samples. The handheld FLIR Airtec monitor was developed to provide real-time DPM data and allow immediate decision making. The monitor works on a laser extinction principle to measure EC, the black component of DPM, as mass accumulates on a filter. The Airtec has proven useful for personal monitoring and short-term DPM surveying. However, capabilities are needed for continuous, long-term monitoring. Continuous DPM monitoring would be highly valuable for applications such as design and operation of ventilation on demand systems, or engineering studies of new ventilation, exhaust treatment or other DPM controls.
The work presented in this thesis considers three continuous monitors, two of which are already commercially available: Magee Scientific's AE33 black carbon (BC) Aethalometer and Sunset Laboratory's Semi-Continuous OCEC Field Analyzer. The third monitor, called the Airwatch, is still in development. The AE33 and Airwatch effectively operate on the same principle as the Airtec, but include a self-advancing filter tape to allow autonomous operation over relatively long periods of time. The OCEC field monitor is essentially a field version of the laboratory analyzer used for traditional 5040 Method analysis. The AE33 has been briefly demonstrated in mine environments in a couple of other studies, but further testing is needed. The current prototype of the Airwatch and the OCEC field monitor have never been mine-tested.
Two separate studies are reported here. The first is a field study in an underground stone mine that tested the Airwatch prototype and AE33 head-to-head under relatively high DPM conditions. Results demonstrated that both instruments could track general trends, but that further work was needed to identify and resolve issues associated with use of both instruments in high-DPM environments – and with basic design elements of the Airwatch. Additionally, the need to calibrate the monitors' output data to the standard measure of EC (i.e., 5040 Method EC) was made clear.
In the second study, laboratory testing was conducted under very controlled conditions to meet this need, and another round of field testing was also done. The second study also included the OCEC field monitor. The laboratory tests yielded data to allow interpretation of the AE33 and Airwatch results with respect to 5040 EC. These tests also shed light on the current range EC concentrations over which these monitors can provide reliable data – which is indeed a primary range of interest for mines. As expected, the OCEC field monitor was shown to produce lab-grade results across a wide range of concentrations. The field testing in the second study demonstrated that all three monitors could operate autonomously in a mine environment over extended periods of time (i.e., weeks to months). Overall, it can be concluded that the AE33 and OCEC field monitor represent off-the-shelf options for DPM monitoring in mines, and the Airwatch might be another option if fully developed in the future. Selection of a particular monitoring tool should include careful consideration of specific factors including data quality needs, conditions in the intended monitoring location(s), and general user friendliness of the monitor.